Xiao Liu PhD Thesis.pdf - Faculty of Information and Communication ...

fine-grained temporal constraints given the input of coarse-grained temporal
constraints by service users [16, 55].
For cloud workflow systems, the quality of temporal constraints can be
measured by at least two basic criteria: 1) well balanced between user requirements
and system performance; 2) well supported for both overall coarse-grained control
and local fine-grained control. Generally speaking, there are two basic ways to
assign QoS constraints, one is task-level assignment and the other is workflow-level
assignment [98]. Since the whole workflow process is composed of individual tasks,
an overall workflow-level constraint can be obtained by the composition of tasklevel
constraints. On the contrary, task-level constraints can also be assigned by the
decomposition of workflow-level constraints. However, different QoS constraints
have their own characteristics and require in-depth research to handle different
scenarios. Currently, the problem of temporal constraint setting in scientific cloud
workflow systems has not been well investigated.
2.4 Temporal Consistency Monitoring
After temporal constraints are assigned at build time, a cloud workflow instance is
required to be under constant monitoring against temporal violations along the entire
workflow execution at runtime. At present, temporal checkpoint selection and
temporal verification are the two basic steps for runtime monitoring of temporal
consistency states [15]. Temporal checkpoint selection is to select specific activity
points along cloud workflow execution to conduct temporal verification on the fly
[18]. As for temporal verification, it is to check the current temporal consistency
state of cloud workflow execution according to the pre-defined temporal consistency
model [17].
In recent years, many checkpoint selection strategies, from intuitive rule based to
sophisticated model based, have been proposed [34, 64, 105]. The work in [34] takes
every workflow activity as a checkpoint. The work in [64] selects the start activity as
a checkpoint and adds a new checkpoint after each decision activity is executed. It
also mentions a type of static activity point which is defined by users at build-time
stage. Since temporal verification inevitably produces extra cost, a Checkpoint
Selection Strategy (CSS) aims to dynamically select only necessary and sufficient
18